Re: [RFC][Patch v1 3/3] PCI: Limit pci_alloc_irq_vectors as per housekeeping CPUs

Nitesh Narayan Lal
2020-09-22T13:55:23Z

[-- Attachment #1.1: Type: text/plain, Size: 3173 bytes --]

On 9/10/20 3:31 PM, Nitesh Narayan Lal wrote:
> On 9/10/20 3:22 PM, Marcelo Tosatti wrote:
>> On Wed, Sep 09, 2020 at 11:08:18AM -0400, Nitesh Narayan Lal wrote:
>>> This patch limits the pci_alloc_irq_vectors max vectors that is passed on
>>> by the caller based on the available housekeeping CPUs by only using the
>>> minimum of the two.
>>>
>>> A minimum of the max_vecs passed and available housekeeping CPUs is
>>> derived to ensure that we don't create excess vectors which can be
>>> problematic specifically in an RT environment. This is because for an RT
>>> environment unwanted IRQs are moved to the housekeeping CPUs from
>>> isolated CPUs to keep the latency overhead to a minimum. If the number of
>>> housekeeping CPUs are significantly lower than that of the isolated CPUs
>>> we can run into failures while moving these IRQs to housekeeping due to
>>> per CPU vector limit.
>>>
>>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>>> ---
>>> include/linux/pci.h | 16 ++++++++++++++++
>>> 1 file changed, 16 insertions(+)
>>>
>>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>>> index 835530605c0d..750ba927d963 100644
>>> --- a/include/linux/pci.h
>>> +++ b/include/linux/pci.h
>>> @@ -38,6 +38,7 @@
>>> #include <linux/interrupt.h>
>>> #include <linux/io.h>
>>> #include <linux/resource_ext.h>
>>> +#include <linux/sched/isolation.h>
>>> #include <uapi/linux/pci.h>
>>>
>>> #include <linux/pci_ids.h>
>>> @@ -1797,6 +1798,21 @@ static inline int
>>> pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
>>> unsigned int max_vecs, unsigned int flags)
>>> {
>>> + unsigned int num_housekeeping = num_housekeeping_cpus();
>>> + unsigned int num_online = num_online_cpus();
>>> +
>>> + /*
>>> + * Try to be conservative and at max only ask for the same number of
>>> + * vectors as there are housekeeping CPUs. However, skip any
>>> + * modification to the of max vectors in two conditions:
>>> + * 1. If the min_vecs requested are higher than that of the
>>> + * housekeeping CPUs as we don't want to prevent the initialization
>>> + * of a device.
>>> + * 2. If there are no isolated CPUs as in this case the driver should
>>> + * already have taken online CPUs into consideration.
>>> + */
>>> + if (min_vecs < num_housekeeping && num_housekeeping != num_online)
>>> + max_vecs = min_t(int, max_vecs, num_housekeeping);
>>> return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
>>> NULL);
>>> }
>> If min_vecs > num_housekeeping, for example:
>>
>> /* PCI MSI/MSIx support */
>> #define XGBE_MSI_BASE_COUNT 4
>> #define XGBE_MSI_MIN_COUNT (XGBE_MSI_BASE_COUNT + 1)
>>
>> Then the protection fails.
> Right, I was ignoring that case.
>
>> How about reducing max_vecs down to min_vecs, if min_vecs >
>> num_housekeeping ?
> Yes, I think this makes sense.
> I will wait a bit to see if anyone else has any other comment and will post
> the next version then.
>

Are there any other comments/concerns on this patch that I need to address in
the next posting?

--
Nitesh

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]